The present invention relates to a centralized controller for controlling the voltage applied to a load having power factor correction capacitors, in particular, for lighting engineering applications. The controller has a power regulator in the form of a converter, which includes a plurality of switching devices located between the AC power supply network or grid and the load.
Street lighting systems, or those systems provided for large sport, industrial, commercial or educational areas, should be able to change their lighting power depending on their real utilization requirements, so as to obtain a considerable saving of electric power and extend lamps life.
Common systems for reducing the lighting power are based on electromagnetic devices, such as transformers with sockets or VARIAC, or on the use of electronic controllers. Both cases are subject to utilization restrictions. In the instance of electromagnetic controllers, these restrictions are basically ascribable to the dimensions and weights of the transformers; a further technical restriction is due to the limited number of applicable sockets connected to a transformer, hindering in practice the possibility of controlling the supply voltage output with a higher precision than a few percentage points. In the case of electronic controllers, two different techniques are used, i.e. the former providing generation of the output voltage through the gating of a set of amplitude modulated voltages impulses (PWM), the latter acting through application of a portion of the supply voltage to the load by means of the so-called phase control technique or also phase cut-off. Both solutions require sudden changes in the voltage applied to the load, which hinder their use in the case of improved loads, i.e. loads fitted with power factor optimizing capacitors, which is a usual condition for high performance lamps with very low power factors.
So far, electronic solutions have required the removal of the capacitive behaviour of the load, or removal of the power factor correction capacitors themselves, i.e. a method adopted when using phase cut-off controllers requiring a current increase in the system, or introducing appropriate filters, i.e. the method adopted for PWM controllers, which again presents the problems of dimensions and weights as is typical for the electromagnetic solutions.
U.S. Pat. No. 4,456,855 discloses a light regulator, especially a light regulator which is suitable for the regulation of the light intensity of both compensated and uncompensated illuminators connected to an alternating-current mains, said regulator having one or a plurality of switch elements fitted in a circuit between the mains and the illuminator, as well as a control circuit for a switch element or elements which in the course of the regulation intensity switches off the supply current at least once on each half cycle when the current has been conducted from the 0-moment of a half cycle to the moment according to a set value.
The main disadvantage of U.S. Pat. No. 4,456,855 is that it does not take into consideration the different behaviour of loads of different nature, thus it does not permit a good regulation strategy.